Anonymrs: For the costumes, put one in a black top, white pants/skirt and the other oppposite. If you’re handy with a needle, you can cut a shirt and pants/skirt (don’t know if you have boys, girs, or both) in half and re-sew the pieces together so the left is white and the right is black or vice versa.
I also thought of black and white cookies. Can you buy jelly beans separately? If so, you can get black and white ones. Licorice. White chocolate.
If I think of anything else, I’ll get back to you.